1. Field of the Invention
This invention relates to the field of disease treatment and prophylaxis. More particularly it relates to the treatment and prophylaxis of Toxoplasma gondii infections.
2. Description of the Background
Toxoplasmosis is caused by the parasitic protozoan, Toxoplasma gondii. In humans, the disease is traditionally associated with the developing fetus in whom it can cause severe neurological problems manifesting as hydrocephaly, mental retardation and/or blindness [1, 2]. In healthy adults, the disease is typically mild, producing few if any symptoms. In immunocompromised adults, however, the parasite can cause severe or even fatal disease [3, 4, 5]. The disease also occurs in other mammals and is a leading cause of spontaneous abortion in sheep.
The parasite itself is extremely widespread and is typically acquired through the ingestion of undercooked meat in which tissue cysts containing the parasite may reside. This form is highly infectious if the meat is not well cooked. Alternatively, the parasite can be contracted through ingestion of foods contaminated with oocysts that are shed in the feces of infected cats. The oocyst is the product of the complete sexual cycle. The oocyst form is highly resistant to destruction by natural elements and can persist in the soil for more than one year after excretion by the cat. In the U.S.A., serological studies indicate that about 10-50% of the population has had contact with the parasite, the prevalence depending on the geographic locales and ethnic group [21]. In countries where eating lightly cooked or raw meat is more common, this figure can rise to as much as 85% (e.g., in France [7]). The incidence of disease in the developing fetus is, fortunately, not as high as these figures might at first suggest because it appears that the fetuses of women who are infected for a significant period of time prior to becoming pregnant are generally not at risk [21].
Diagnosis of congenital infection has in the past relied on serology (reviewed in [1,21]). This can be done postnatally or, ideally, pre-natally and relies on the relative titers of IgG and IgM (to deduce whether the titers are due to a current infection or legacy of a past infection). The factors contributing to the severity of disease in the developing fetus have been poorly understood. The only well-established factor is that the time of initial infection of the mother relative to conception is critical: infection significantly before conception such that an effective immune response has been mounted by the mother, results in little if any fetal disease. Infection immediately before or after conception (i.e., in the first trimester of pregnancy) results in severe disease for about 10-15% of fetuses [21].
In the past two decades, toxoplasmosis has dramatically increased in a relatively new group of patients who are in some way immunodeficient as a result of post-transplantation therapy [5, 9, 10, 22], neoplastic disease [11, 12, 13, 22] or, most recently, acquired immunodeficiency syndrome (AIDS) [3, 4, 5]. In such immunodeficient patients, the parasite can cause a disseminated, potentially fatal form of the disease [5, 22, 27].
Typical AIDS patients with toxoplasmosis exhibit signs referable to the central nervous system as the first symptom of the disease (reviewed in [22]), as one of the tissues most affected by the parasite is the brain, where massive numbers of parasites and of tissue cysts can be found. Infection is not limited to the brain, however, and parasites and tissue cysts can be found throughout the body [11]. The typical routine for diagnosis includes serology, computed tomography, magnetic resonance imaging and/or brain biopsy [1, 15, 16]. Of these, the only definitive route to diagnosis is the brain biopsy as this enables the direct visualization of the parasite, using immunoperoxidase staining [17].
In almost all AIDS patients and in most cases of toxoplasmosis in cancer patients and renal transplant recipients, toxoplasmosis results from a recrudescence of a previous latent (i.e., chronic) Toxoplasma infection. In contrast are patients at risk for the acute acquired infection, such as the fetus of a previously uninfected pregnant woman or a previously uninfected organ transplant recipient who receives an organ from a sero positive (i.e., Toxoplasma-infected) donor.
In general, there are three types of therapy: acute therapy, primary prophylaxis, and secondary prophylaxis. Acute therapy refers to treatment during an acute phase of an infection. In certain severely immunocompromised patient groups, this is followed by secondary prophylaxis (also known as maintenance therapy), which may be given over the entire life of a patient. Primary prophylaxis refers to treatment given to prevent the infection from occurring. Primary prophylaxis is often used in heart transplant recipients who are seronegative and who receive a heart from a seropositive donor. Primary prophylaxis is also used in pregnant women to prevent transmission from the mother to the fetus; that is, treatment is intended to prevent the mother who acquired the acute infection during pregnancy from passing the parasite to her fetus, as well as to treat the fetus in utero. Primary prophylaxis is also frequently used in AIDS patients to prevent activation of their latent (chronic) toxoplasma infections.
The course of treatment for toxoplasmosis in pregnant individuals is determined by the stage in pregnancy and whether the infection is acute or chronic. The purpose of early treatment is to attempt to prevent transmission of the parasite to the fetus. However, the fetus may be treated by treating the mother during gestation. If infection is acute, the antibiotic spiramycin may be administered but is of unproven efficacy. More effective drugs such as pyrimethamine and sulfadiazine, especially when used in combination, are often used after the first trimester of pregnancy (pyrimethamine may be teratogenic) when the diagnosis of infection of the fetus has been established by prenatal diagnostic techniques. Otherwise this particular drug combination is generally not used during pregnancy because of the potential toxicity for the mother and for the developing fetus [21].
Treatment of toxoplasmosis in non-pregnant individuals is initiated and maintained with a drug regimen involving a combination of folate antagonists, such as pyrimethamine and sulfadiazine [1, 14]. If the disease is identified soon enough, treatment is reasonably effective in combatting the acute disease. However, due to poor tolerance of the drugs, especially of the sulfa compounds in AIDS patients, maintenance on the drug therapy is frequently not possible, and recrudescence of the infection is often observed (that is, the drug therapy reduces but does not eliminate the parasite infection).
Rifamycin compounds are macrocyclic antibiotics that have been shown to be useful in a number of selective therapeutic applications. For example rifampin has the following structure: ##STR1## However, rifampin was tested for effectiveness in treating toxoplasmosis and was shown to have no protective effect in mice challenged with a lethal inoculum of toxoplasma [18]. Rifampin at concentrations of 50 .mu.g/ml and greater significantly inhibited multiplication of toxoplasma in L-cell cultures. However, similar concentrations also inhibited growth of L-cells. Because the toxicity of rifampin for L-cells and its inhibition of Toxoplasma multiplication intracellularly in vitro occurred at the same concentration of the drug, it was reported [18] that rifampin was likely to inhibit Toxoplasma multiplication by its toxic effect on the L-cells. On the other hand, rifamycin compounds are generally considered to be effective against a limited number of pathological organisms, generally Gram-positive bacteria (including mycobacteria, staphylococci, and streptococci) and some Gram-negative bacteria (e.g., Brucella, Chlamydia, Haemophilus, Legionella, and Neisseria spp); other Gram-negative bacteria (e.g., enterobacteria) are less sensitive, and spirochaetes and mycoplasma are known to be insensitive to treatment [19]. Individual compounds within the rifamycin series can be quite specific in their clinical indications. For example, the Physicians Desk Reference (1993 edition) lists only tuberculosis and asymptomatic infection with N. meningitidis as indications for treatment with rifadin. The related compound rifabutin, which is a spiropiperidyl rifamycin derivative, has also primarily been used in treating mycobacterial infections, notably tuberculosis [20]. More recently it has been used for primary prophylaxis of Mycobacterium avium-intracellulare infections in patients with AIDS [25]. However, these uses have been directed to bacteria, not protozoans which are members of the animal kingdom).
Accordingly, there remains a need for the development of therapeutic and prophylactic methods that can measurably add to the reliability of disease reduction in toxoplasmosis. By adding to the spectrum of drugs available for treating toxoplasmosis, problems arising from parasite resistance and side effects of existing medications can be overcome.